The present invention relates, in general, to lead frames for semiconductor device packages, and more particularly, to a novel method of removing contaminants from a lead frame.
In the past, lead frames that were used to produce semiconductor device packages had contaminants that made it difficult to produce reliable semiconductor device packages. Typically, lead frames were plated with a material, such as nickel, to protect the lead frame from corrosion. This plating oxidized to nickle oxide which improved adhesion of encapsulation material to the lead frame, and also improved adhesion of wire bonds and die bonds to the lead frame. During plating operations, the lead frames were immersed in plating baths or solutions. As these solutions were repeatedly used, they gradually became contaminated with phosphorus, chlorine, and other chemicals. These chemicals were applied to the lead frame along with the plating material and formed contaminants that were dispersed throughout the plated material. Since contaminants in the plating solution varied over time, contaminant concentrations in the lead frame's plating material also varied making it difficult to develop standard die bonding, wire bonding, and encapsulation procedures that consistently produced reliable results.
In addition to plating contaminants, hydrocarbons and other contaminants developed on the lead frame's surface. These contaminants resulted from a variety of sources including body oils from humans handling the lead frames, machine oils from machines handling the lead frames, and from other contaminants in the environment.
Previous methods for cleaning lead frames reduced some of the contaminants' concentration, but, handling of the lead frames during assembly operations could reintroduce some contaminants. Additionally, the previous cleaning methods generally left chemical residues, such as phosphorus, on the lead frames which added to the contaminants. Due to the variation of contaminants in the plating baths, residues from cleaning operations, and the variety of handling operations, the amount of contamination varied between lead frames from one production run to the next. Such variations made it difficult to establish packaging assembly procedures that could accommodate the lead frame's varying contaminant concentrations.
Accordingly, it is desirable to have a method of removing contaminants from lead frames which elimiinates contaminants from lead frame surfaces, which eliminates contaminants from plating material on the lead frames, and which provides a consistently low level of contaminants thereby permitting consistent and repeatable assembly procedures to be established.